Water models four-site

The number of sites reflects the possibility we have examined, and advocated, of using many-center expansions to improve the representation. Each expansion center will be a site. There are models with one, two, and more sites. This sequence of increasing complexity reaches the number of heavy atoms of the molecule and then the whole number of atoms, including hydrogens. It is not limited to the nuclei as expansion sites. There are potentials introducing other locations of sites, in substitution or in addition to the nuclei. For example potentials widely used in simulations adopt for water a four-site model other potentials (rarely used in simulations) prefer to use the middle of the bonds instead of (or in addition to) nuclei. Each site of the molecule must be combined with the sites of the second (and other) molecule to give the potential. 

H.W. Horn et al., Development of an improved four-site water model for biomolecular simulations TIP4P-Ew. J. Chem. Phys. 120, 9665 (2004)... 

Mahoney MW, Jorgensen WL. A five-site model for liquid water and the reproduction of the density anomaly by rigid, nonpolar-izable potential functions. J. Chem. Phys. 2000 112 8910-8922. Rick SW. A reoptimization of the five-site water potential TIP5P for use with Ewald sums. J. Chem. Phys. 2004 120 6085-6093. Horn HW, Swope WC, Pitera JW, Madura JD, Dick TJ, Hura GL, Head-Gordon T. Development of an improved four-site water model for bio-molecular simulations UP4P-Ew. J. Chem. Phys. 2004 120 9665-9678. 

Figure 9.3 Multisite models of water (a) three-site, (b] four-site, (c) five-site.

A major application of molecular level simulations of water is to study the properties of solutions. For this reason, a considerable amount of effort has gone into developing models capable of describing the interactions of water molecules both with other water molecules and with various solute molecules. One important factor in this effort was to keep the number of interaction sites small, because the computational effort scales with the square of the number of sites per molecule. Hence, emphasis was placed on three- and four-site models (summarized in Table 6) rather than on more elaborate schemes. 

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